Piezoelectric crystal oscillator



Sept. 13, 1938. w. A. FORD 2,130,272

PIEZOELECTRIC CRYSTAL OSCILLATOR Filed March 51, 1938 a FI6.Z.

v I l I I I I I I l I l I I l I l I I" 53 e2 7/ so as 58 ma 1/? CAPACITY C mrrafd I 2/ 24 ,7 w F|6.3. Z7 22 1 M 25 U 1 I l lllllflllimlllllllllll 26 1 Inventor" War "Pen A. F-or'd,

His Attorney.

Patented se t. 13, 1938 2,130,272

UNITED STATES PATENT OFFICE PIEZOELECTRIC CRYSTAL OSCILLATOR Warren A. Ford, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application 'March 31, 1938, Serial No. 200,191

7 Claims. (01. 25036') My invention relates to electric valve oscilto the cathode I9. A condenser '26, connected lators and more particularly to a piezo electric across the terminals of the 'high voltage source crystal controlled electric valve oscillator. I8, maintains the positive terminal at ground Vacuum tube oscillators are commonly propotential for oscillations of the frequency of the vided with a piezo electric crystal which acts as oscillatory circuit I4, I5. e '5 a control element to maintain the frequency The electron discharge device I! is provide of the oscillator substantially constant during with an "anode 2| which is connected by a conoperation. Such crystals are subjected during ductor 22 to a point on the high voltage source normal operation of the oscillator to high volt- I8 of'a 'potentiallower than that supplied to the 10 ages, and, at times, tophysical vibration and grid I6. A condenser 23 maintains the anode 10 jars which cause the crystal to crack, chip, or 2| at ground potential for oscillations of the otherwise to'break down electrically and thereby frequency of the oscillatory circuit I4, I5. to become inoperativeand incapable of perform- A second grid 24 of the electron discharge deing their intended function. It is preferable vice I! is negatively biased from a sourceofpoat such times that the oscillator shall continue tential '26 through a high resistor 25, the re- 15 after break down of the crystal to perform with sister being provided to maintain the grid 24 as nearly an unimpaired operation as possible. above ground potential for oscillations of the It is an object of my invention to provide a frequency of the oscillatory circuit I4, I5. A crystal controlled oscillator which is capable of third grid 2'! provided in the electron discharge 7 a high degree of operability after break down of device I1 is connected-directly to the cathode I9. 20 the controlling crystal. My oscillator in'operation is under control of A further object of my invention is to provide a resonant device or circuit having a constant a crystal controlled oscillator having extreme frequency characteristic. The resonant device stability with the advantages of simplicity and may, for example, take the form of a piezoelec- 25 unusual reliability. tric crystal 29 which is connected between the 25 A further object of my invention is to progrid I6 and the grid of the electron discharge vide an oscillator operating, under control of a device I! to control the operation of the oscilcrystal, with a principle of operation quite similator in'a mannernow to be explained.

lar to that of a dynatron oscillator and one which Assume that the oscillatory circuit I4, I5 is 30 therefore performs with a high degree of unituned substantially to the natural frequency, 30 'formity. or a desired harmonic frequency, of the crystal The novel features which I'believeftobe char- 29. A rise in the voltage across the oscillatory acteristic :of my invention areset forth with parcircuit I4, I5, is then transmitted by the crystal 'ticularity-in'the appended'claims. Myinvention 29 to the grid 24 which, being maintained by 5 itself, however, both :as to its organization-and the resistor 25 above ground potential for al- 5 method of operation, together with further obternating currents, thereupon has its potential jects and advantages thereof, may best be unmade less negative. The grid 24 is physically dBISIJOOd y vreference the following ppositioned between the grid I6 and the anode 2| tion taken in connection with the accompanyand, being negative t respect t th th 40 t drawing in which 6 P- grid I6 and the anode 2|, normally operates to 40 bodimfmt of my mvfmtlon; Flgi11ustr repel and turn back some of the electrons which graphlfzany Ian operatmg ,charactenstlc i thls have passed from the cathode I9 through the imbqdlmenti a 3 mustrates mven' grid I6. These repelled electrons are attracted m "i F to the grid is. When the grid 24 is made less Ref.ernr 1g q i fi y to 1 of negative by a potential transmitted through the drawing, my invention is illustrated .as em bodied in an oscillator used to supply energy to crystal. fewer electrons are turned back 1; tit i it; i biaiiliiiifiifiill? 633K251? rm r rans ormer' as a rima iy fi .vconneitedi with .aicondensgr 2 a rent of the grid I6 thereupon decreases. It 0 parallel-tuned oscillator-y circuit. .A grid I6 Jof Wmthus be l l' 4 9 ease of potemial the electron rdischarge 'devicze ,is connected across osclllatory Clrcult results in a. through the oscillatory circuit .I'4, I5 ito the 6 61 1? 1 3 h d i h i fil i t thls positive terminal'o-f :"a source tof .high potential 0 13111 e 8 S mega 1V8 r8515 i 1-8, the negative terminal of .WhlCh'JiS connected characteristic exhibited bythe'electron discharge 55 device I! is conducive to and results in sustained oscillations in the oscillatory circuit 4, I5.

Consider now the effect on this operation of the piezo electric crystal 29. The crystal has a sharply peaked transmission characteristic at its natural frequency and a similar though minor transmission characteristic at each of its harmonic frequencies. The potential drop across the crystal is thus a minimum for an oscillating potential having a frequency substantially that of the natural frequency, or a harmonic frequency, of the crystal but is comparatively high for oscillating potentials of other frequencies. It will then be clear that the crystal 29 transmits a maximum voltage from the oscillatory circuit 4, l5 to the grid 24 when the oscillatory circuit |4, I5 is resonant at a frequency substantially that of the crystal or a harmonic thereof. Conversely, decreasing values of oscillatory potentials are impressed upon the grid as the oscillatory circuit |4, I5. is detuned away from the natural frequency of the crystal 29. This results in a tendency of the oscillator to lock in step with and oscillate at the frequency of the crystal 29 whenever the oscillatory circuit l4, i5 is tuned closed to the frequency of the crystal since, in this condition of'operation, the oscillatory voltage transmitted through the crystal 29 to the grid 24 attains a maximum value.

Fig. 2 illustrates graphically the operating characteristic of my invention in the embodiment shown in Fig. 1. This is a graph of the frequency departure in cycles of the oscillator output frequency from the natural frequency of the crystal,

" which in this case was 4206 kc., as the condenser I5 was varied from its minimum setting to its maximum setting. It will be seen from this graph that the oscillator frequency remains substantially unchanged while the condenser is varied from 53 mini. to 98 mmf., the frequency departure throughout this range varying from 1300' to 1400 cycles, a frequency departure of only .03 percent. The piezo electric crystal 29 lost con trol when the condenser |5 was varied beyond the limits of 53 mmf. and mmf, an adjustment to a capacity slightly less than 53 mmf., causing the output frequency immediately to jump to an oscillating frequency of 5400 kc. while an adjustment to a capacity slightly more than 115 mmf., resulted in the oscillatory circuit I4, I5 breaking into oscillation at a frequency 3560 kc. The proper operating frequency of an oscillator having the operating characteristics here shown is perhaps within the range between a condenser setting of 98 mmf. and 108 mmf. A failure of the piezo electric crystal 29 when the setting is within the range mentioned results, as explained above, only in a slight shift in the oscillator frequency to a new frequency determined by the particular tuning of the oscillatory circuit l4, l5.

The grid 21 is directly connected to the cath ode in the arrangement of Fig. 1 and, as thus connected, has no effect on the operation of the oscillator.

A modification of my invention is shown in Fig. 3 wherein elements corresponding to like elements of Fig. l are designated by like reference characters. In this modification, the crystal 29 of Fig. 1 has been replaced by a series resonant circuit comprised by the condenser 30 and'inductance 3|. The oscillatory circuit M, I5 is tuned to a frequency determined by the values ofcapacity and inductance in the condenser 30 and inductance 3| respectively. The series resonant circuit 30, 3| preferably is sharply resonant at the desired frequency chosen for the operation of the oscillator and controls the operation by controlling the feed back potential from the oscillatory circuit l4, I5 to the grid 24 in the same manner as does the crystal 29 of Fig. 1. The condenser 30 and inductance 3| are preferably enclosed in an atmosphere maintained under constant conditions of temperature and humidity.

In this modification the grid 21 is shown as provided with a small negative bias from a source of biasing potential 28. The effect in providing this grid with a negative bias is that the grid now controls the magnitude of the electronic flow from the cathode and thereby controls the magnitude of the effect produced by the other elements of the electron discharge device I! dur ing the operation of the oscillator.

The operation of the Fig. 3 embodiment of my invention is the same as that of Fig. 1 and will, therefore, not be explained in detail.

My oscillator arrangement has the very desirable advantage that should a piezo electric crystal be used as a controlling element and should the crystal break down or become inoperative, the oscillator continues to oscillate at substantially its former frequency. This desirable operation is the result of a certain amount of the oscillatory potential from the circuit |4, |5 being transmitted to the grid 24 through the inherent capacity existing between the elements of the crystal holder and associated wiring. The magnitude of the potential thus capacitively transmitted to the grid 24 may be regulated by the size and arrangement of the elements of the crystal holder and i associated wiring. This operating characteristic of my oscillator is especially to be desired in certain types of service as, for example, in aircraft communication where consistent communication between a plane and ground is of utmost importance.

While I haveshown a particular embodiment of my invention, it will of course be understood that I do not wish to be limited thereto since many modifications may be made both in the circuit arrangement and in the instrumentalities employed, and I therefore contemplate by the appended claims to cover any such modifications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A vacuum tube oscillator comprising, in combination, an electron discharge device having an anode, a cathode, and a plurality of grids, a tunable frequency determining network connected between a first of said grids and said cathode, means included in said connection for impressing a positive potential on said first named grid, means for impressing a negative potential on a second of said grids, means for impressing a. positive potential on said anode of a value less than that on said first named grid, and means for producing sustained oscillations in said tunable network, said last named means including means resonant .at a predetermined frequency for supplying an alternating current potential of substantially said predetermined frequency from said first named grid to said second named grid.

2. An oscillator comprising, in combination, an electron discharge device having an anode, a cathode, a first grid, and a second grid positioned between said first grid and said anode, a tunable oscillatory circuit connected between said first named grid and said cathode, means included in 1 said connection for impressing a positive potential on said first named grid, means for impressing a negative potential on said second named grid, means for impressing a positive potential on said anode, said anode potential being less than that impressed on said first named grid, an output circuit associated with said oscillatory .circuit, and means for producing sustained oscillations in said tunable oscillatory circuit, said last named means including frequency responsive means resonant at a fundamental and one or more harmonic frequencies for supplying an alternating current potential of substantially the fundamental or harmonic frequency determined by said frequency responsive means from said first named grid to said second named grid.

3. The combination, in a vacuum tube oscillator, of an electron discharge device having physically spaced in the order named an anode, a first grid, a second grid, a third grid, and a cathode, a tunable frequency determining network connected between said second grid and said cathode, means in said connection for impressing a positive potential on said second grid, means for impressing a negative potential on said third grid, means including an impedance connected between said first grid and said cathode for impressing a negative potential on said first grid, means for impressing a positive potential on said anode, said anode potential having a value less than the potential supplied to said second grid, means resonant at a fundamental frequency, and means including said last named means for supplying an alternating current potential of substantially said fundamental frequency from said second grid to said first grid.

4. A vacuum tube oscillator comprising, in combination, an electron discharge device having spaced in the order named an anode, a first grid, a second grid, and a cathode, a resistor connecting said first grid to said cathode, means included in said connection for biasing said first grid negatively, a tunable oscillatory circuit connected between said second grid and said cathode, means included in said last named connection for impressing a positive potential on said second grid, means for impressing a positive potential on said anode, said anode potential having a value less than the potential on said second grid, a series resonant circuit having a fundamental frequency, and means including said series resonant circuit for supplying an oscillatory potential having substantially said fundamental frequency from said second grid to said first grid whereby sustained oscillations are produced in said tunable oscillatory circuit.

5. The combination, in an electric valve oscillator, of an electron discharge device having an anode, a cathode, and a plurality of grids, a tunable frequency determining network connected between a first of said grids and said cathode, means included in said connection for impressing a positive potential on said first grid, means for impressing a negative bias on a second of said grids, means for impressing a positive potential on said anode of a value less than the positive potential on said first named grid, and means for producing sustained oscillations in said frequency determining network, said last named means including a piezo electric crystal for supplying an oscillatory potential from said first named grid to said second named grid.

6. A vacuum tube oscillator comprising, in combination, an electron discharge device having physically spaced in the order named an anode, a first grid, a second grid, and a cathode, a resistor connected between said first grid and said cathode, means included in said connection for biasing said grid negatively, a parallel-tuned circuit connected between said second grid and said cathode, means included in said last named connection for impressing a positive potential on said second grid, means for impressing a positive potential on said anode, said anode potential having a value less than the potential of said second grid, and means including a piezo electric crystal having a natural frequency substantially that of said parallel-tuned circuit for supplying an alternating current potential from said second grid to said first grid.

7. A vacuum tube oscillator comprising, in combination, an electron discharge device having physically spaced in the order named an anode, a first grid, a second grid, and a cathode, a resistor connected between said first grid and said cathode, means included in said connection for biasing said grid negatively, a parallel-tuned circuit connected between said second grid and said cathode, means included in said last named connection for impressing a positive potential on said second grid, means for impressing a positive potential on said anode, said anode potential having a Value less than the potential of said second grid, and means including a piezo electric crystal having a harmonic frequency substantially equal to the frequency of said parallel-tuned circuit for supplying an alternating current potential from said second grid to said first grid.

WARREN A. FORD. 

